Are you guys sure? I see on the wikipedia list that Borg became number 1 on august 23rd 1977 and held that position for just 1 week.
McEnroe also got one week in 1980 - aug 11th. And there were five more "one-weekers" in 1982 and 1983, courtesy of Connors and McEnroe.
They did occasionally release rankings two or more weeks in a row, just not regularly.